1. Field of the Invention
The present invention relates to an electrically heated catalytic converter system for an engine that has a catalytic converter equipped with an electric heater, and more specifically, relates to such a system applied to an engine having a plurality of exhaust passages.
2. Description of the Related Art
An exhaust gas purification device that utilizes a three-way reducing and oxidizing catalytic converter disposed on the exhaust passage of the internal combustion engine is commonly used. Generally, the catalyst used in such converters is able to purify the pollutants in the exhaust gas only when the temperature of the catalyst becomes higher than a certain temperature, i.e., the catalysts in the converter do not work when the temperature of the catalyst is lower than an activating temperature.
Usually, the catalyst in the converter is gradually heated by the exhaust gas of the engine and reaches the activating temperature once the engine starts. However, when the temperature of the engine is low, such as during a cold start, it takes a substantially long time to heat up the catalyst to the activating temperature since the heat of the exhaust gas is taken by the cold wall of the exhaust passage before reaching the converter. Therefore, in the cold start of the engine, the exhaust gas of the engine is not sufficiently purified since the temperature of the catalyst is lower than the activating temperature.
To solve this problem, Japanese Unexamined Utility Model Publication (KOKAI) No. 4T-22313 discloses a catalytic converter which is equipped with an electric heater for heating up the catalyst during starting up of the engine. In this device, the electric heater is switched on when the engine is started to heat up the catalyst so that the temperature of the catalyst reaches the activating temperature in a short time.
Even in the electrically heated catalytic converter system, the emission in the exhaust gas during start-up is worsened, if the converter system, especially, the electric heater, fails. However, in the system such as disclosed in Japanese Unexamined Utility Model Publication (KOKAI) No. 4T-22313, the operation of the engine is not affected even if the converter has failed. Therefore, the operator of the engine may not notice the failure of the converter even if the failure actually occurs. In this case, the engine is operated with a failed heater, and thus with increased emissions during the start up of the engine.
Therefore, it is necessary to detect the failure of the converter system to assure its proper operation during the start up of the engine.
However, it is difficult to detect the failure of the converter system accurately in actual use. For example, it may be possible to determine that the converter system has failed if the temperature of the converter does not become higher than a predetermined value even when a predetermined time has lapsed after the electric heater is switched on at the start up of the engine. However, in some cases the temperature of the converter becomes high in a short time after the engine starts due to the heat generated by oxidation of HC and CO in the exhaust gas without being heated by the electric heater. In such a case, the temperature of the catalyst exceeds the predetermined value shortly after the engine start even if the electric heater has failed, and thus the failure of the electric heater cannot be detected.
Also, for example, if the catalyst in the catalytic converter deteriorates, or the resistance of the lead wires of the heater increases due to corrosion of the lead wire, the time required to heat up the catalyst becomes longer. In these cases, the time required for heating up the catalyst increases gradually as the deterioration of the catalyst or corrosion in the lead wires proceeds. In such cases, if the failure of the catalyst or electric heater is detected by the above method, i.e., if it is determined that the converter system has failed only when the temperature of the catalyst does not exceed a fixed predetermined value, the failure cannot be detected until the deterioration of the catalyst or the corrosion of the lead wire proceeds to some extent. This may prevent the failures from being detected in early stages of the failure.